The present invention is directed to an arrangement of optical waveguides, which arrangement has at least two optical waveguides with each having an input side for coupling an optical wave into the waveguide and an output side for coupling out an optical wave conducted in the waveguides and each waveguide having a predetermined length between the two ends, and means for producing a modification of the optical length of the waveguide so that one waveguide can have a smaller optical length than the other waveguide.
U.S. Pat. No. 5,559,906, whose disclosure is incorporated herein by reference thereto and which claims priority from the same German Application as European Patent Document 0 662 621, discloses an optical arrangement which has at least two optical waveguides, each having an input-side end and an output-side end and each waveguide having a determined optical length between the two ends and means for producing a modification of the optical length of the waveguides so that the means produces an optical length in at least one of the waveguides smaller than the other waveguide.
The decisive optical length of a waveguide in such an arrangement is given by the product nL of an index of refraction n and a geometric length L of the waveguide. The geometric length L of the waveguide is the length of an optical axis extending between the coupling-in end and the coupling-out end of the waveguide, along which the optical wave is propagated in the waveguide. The index of refraction n of the waveguide is given as its effective index of refraction by 2.pi..multidot..beta..multidot..lambda., wherein .beta. is the propagation constant of the optical wave conducted in this waveguide along the optical axis and .lambda. is its wavelength. Once the index of refraction n and the geometric length L of each waveguide of the arrangement is defined, the optical length nL for the waveguide is also defined.
The known arrangements comprise a means with which a modification .DELTA.(nL) of the optical length nL of the waveguides is enabled, despite the defined optical length nL of each waveguide. Such means for the production of the modification .DELTA.(nL) of the optical length nL of the waveguides can be fashioned in such a way that it modifies the index of refraction n of the waveguide and/or its geometrical length L.
For example, a waveguide can comprise a material with an index of refraction that can be modified by a certain physical quantity. The modification .DELTA.(nL) of the optical length nL of this waveguide is, as a rule, greater the larger the absolute value of the physical quantity that modifies the index of refraction and/or the larger the part of the optical length nL of the waveguide is on which this quantity acts, and vice versa.
In U.S. Pat. No. 5,559,906, examples of the electro-optical effect, in which the waveguide comprises an electro-optical material whose index of refraction can be modified by allowing an electrical field to act on the material, were disclosed. The following are cited: charge carrier injection, in which the waveguide comprises semiconductor material whose index of refraction can be modified by an electrical injection of charge carriers into the material, and/or the thermo-optical effect, in which the waveguide comprises a thermo-optical material whose index of refraction can be modified by modification of the temperature.
In these cases, the means for producing a modification .DELTA.(nL) of the optical length nL of the waveguide comprises means for the optical production of an electrical field, charge carriers and/or a temperature modification. A temperature modification can also produce a modification of the geometric length L of the waveguide.
Specifically, all waveguides in the known arrangement comprise an optical length nL different from one another so that this length nL, based on the waveguide of the shortest optical length, increases from waveguide to waveguide in the direction toward a waveguide of the greatest optical length. The different optical lengths nL of the different waveguides can be obtained by means of an index of refraction n differing from one another and/or geometrical length L of the waveguides differing from one another.
If, for example, the different waveguides have the same index of refraction n among themselves, the different optical lengths nL of the different waveguides can be obtained only by a different geometrical length L of these waveguides.
Moreover, the known arrangement is specifically set up so that the output-side ends of the waveguides are arranged with a small spatial distance from one another, so that the optical waves coupled out from these ends are superposed coherently on one another.
Given simultaneous coupling of an optical wave that contains one or more wavelengths differing from one another into all waveguides of this arrangement through the input-side end of these waveguides, each of these waveguides has an optical power portion containing all of these wavelengths of this wave is respectively conducted to the output-side of this waveguide and is coupled out at this end. The power portions coupled out from all output-side ends are coherently superposed on one another.
The mutually differing optical lengths nL of the waveguides and the coherent superposition of the coupled-out power portions provide that the coupled-out optical power portions falling at a single wavelength are concentrated in a spatial point allocated individually to this wavelength, and that the coupled-out optical power portions falling at wavelengths differing from one another are concentrated in spatial points that are spatially at a distance from one another.
In this way, given the use of a large number of optical waveguides, the known arrangement enables a large number of wavelengths differing from one another to be separated from one another, and each of these wavelengths can be a central wavelength of a respective optical wavelength channel.
Using the means for producing a modification of the optical length nL of the waveguides, in the known arrangement, it is possible to spatially displace the spatial point allocated individually to an individual wavelength, in which point the coupled-out optical power portions falling at this wavelength are concentrated. The spatial point can be displaced to such an extent that after this displacement, the spatial point assumes the position of a different spatial point at which, before the displacement, the coupled-out optical power portions of a different wavelength are concentrated, so that after the displacement, the coupled-out optical power portions falling of one wavelength are now concentrated at the location of the other spatial point.
The means for producing the modification .DELTA.(nL) of the optical length of the waveguide is usefully fashioned in such a way that the produced modification .DELTA.(nL) of the optical length nL, beginning with the waveguide with the shortest optical length, increases from waveguide to waveguide in the direction toward the waveguide with the greatest optical length.
For example, in the known arrangement, each waveguide comprises thermo-optical material that can be heated by means of a heating electrode. Above each waveguide, at least one respective heating electrode is present for heating the thermo-optical material, which electrode heats a part of the optical length nL of this waveguide that increases in the one direction from waveguide to waveguide. In this specific embodiment, the known arrangement can be used as a tunable optical grid or filter that enables separation of a large number of wavelength channels from one another, and, given a fixed wavelength, can be used as a 1.times.N switch, and can be used for a wave division multiplex (WDM) transmission, wavelength switching and/or wavelength conversion.
In the known means for producing a modification .DELTA.(nL) of the defined optical length nL of the waveguides, the use of heating electrodes with the exploitation of the thermo-optical effect has the effect that this length nL can only be increased in each waveguide, and thereby the spatial displacement of the spatial point individually allocated to an individual wavelength, in which point the coupled-out optical power portions falling in this wavelength are concentrated, is possible only in one direction.